1. Field of the Invention
The present invention relates to an inter-axle differential, and more particularly, to an improved structure for mounting differential gears in the differential.
2. Discussion of Related Art
Motor vehicles with solidly connected multiple drive axles are commonly equipped with an inter-axle differential when there is any physical requirement for speed difference between the drive axles, e.g., to allow torque balance between the drive axles during the vehicle cornering, to compensate for tire size differences, etc. Inter-axle differentials are widely employed for tandem drive axles of heavy-duty trucks for on- and off-road service as a power divider.
A conventional inter-axle differential includes a spider that is rotatably driven by a power input shaft. The spider includes a plurality of radially outwardly extending arms each of which includes a pinion gear. Rotation of the spider and pinion gears causes a corresponding rotation in side gears that are configured to indirectly transmit power to first and second axles of a vehicle. The pinion gears are typically retained on the spider arms by a hardened differential case. The use of a differential case, however, reduces the accessibility of the pinion gears for required lubrication and also reduces the space available for the gears. Several attempts have been made to eliminate the need for a differential case. See, e.g., U.S. Pat. No. 5,823,908, U.S. Pat. No. 5,897,452, and/or U.S. Pat. No. 6,190,281. In each of these inter-axle differentials, however, the mechanism used to retain the pinion gears on the spider can spin off or become loose due to centrifugal forces acting on the retaining mechanisms. 
The inventors herein have recognized a need for an inter-axle differential that will minimize and/or eliminate one or more of the above-identified deficiencies.